The present invention relates to a variable wavelength semiconductor laser of a narrow linewidth.
The semiconductor laser has already been put to practical use as a light source for optical fiber communication because it is small, highly efficient and highly reliable. Systems heretofore employed take advantage of the direct modulation capability that is one of the outstanding features of the semiconductor laser, and one of such conventional systems is what is called a direct intensity modulation-direct detection (DIM-DD) system in which intensity-modulated light corresponding to the amount of current injected into the semiconductor laser is received directly by a photodiode or avalanche photodiode after being propagated through an optical fiber. A dynamic single wavelength laser which stably operates at a single wavelength even during high-speed modulation, such as a distributed feedback (DFB) semiconductor laser, has been developed for use as the light source in the DIM-DD system, with a view to lessening the influence of dispersion by a single mode fiber so as to increase the repeater spacing.
On the other hand, it is possible to substantially improve the receiving sensitivity and hence increase the repeater spacing more than in the DIM-DD system by positively utilizing the properties of a wave itself of light, such as its frequency and phase. This system is referred to as a coherent transmission system, which is being given much study experimentally as well as in its theoretical aspect and is now considered promising as a future optical communication system (see T. Okoshi, Journal of Lightwave Technology, Vol. LT-2, pp. 341-346, 1984, for example). Because of its property, the coherent transmission system requires that the light source at the transmitting side and the light source as a local oscillator at the receiving side be small in linewidth and variable in wavelength. In studies made so far on a laboratory scale, intended primarily for evaluating the potential of the system, it is customary to attain the highly coherent and tunable output by a gas laser of an extremely narrow linewidth or a more practical ordinary semiconductor laser in which an external diffraction grating is provided and only a specific wavelength is fed back thereby to the laser. Since the light emitting region of the semiconductor laser is as small as about 1 .mu.m in diameter, however, such a structure in which the light source and the external diffraction grating are not formed unitary with each other is readily affected by mechanical vibration and heat variation, unstable in providing desired characteristics and involves a large-scale system configuration; apparently such a laser structure is impractical.
For the reduction of the linewidth it is an effective method to increase the cavity length of the laser. However, conventional semiconductor lasers of the type have not yet a stable narrow linewidth characteristic.